This invention relates to an in-line electron gun for a color picture tube, and particularly to a structure and method for modifying such an in-line gun.
An in-line electron gun is one designed to generate preferably three electron beams in a common plane and direct those beams along convergent paths in that plane to a point or small area of convergence near the tube screen.
A problem that exists in a color picture tube having an in-line gun is a coma distortion wherein the sizes of the three rasters scanned by the three beams on the screen by an external magnetic deflection yoke are different because of the eccentricity of the two outer beams with respect to the center of the yoke.
A number of structures known in the art correct for coma. Messineo, et al., U.S. Pat. No. 3,164,737 issued Jan. 5, 1965, teaches that a coma distortion caused by using different beam velocities can be corrected by use of a magnetic shield around the path of one or more beams in a three gun assembly. Barkow, U.S. Pat. No. 3,196,305, issued July 20, 1965, teaches the use of magnetic enhancers adjacent to the path of one or more beams in a delta gun, for the same purposes. Krackhardt, et al., U.S. Pat. No. 3,534,208, issued Oct. 13, 1970, teaches the use of a magnetic shield around the middle one of three in-line beams for coma correction. Yoshida, et al., U.S. Pat. No. 3,548,249, issued Dec. 15, 1970, teaches the use of C-shaped elements positioned between the center and outer beams to enhance the effect of the vertical deflection field on the center beam. Murata, et al., U.S. Pat. No. 3,594,600, issued July 20, 1971, teaches the use of C-shaped shields around the outer beams with the open sides of the members facing each other. These shields appear to shunt the vertical deflection field around all three beams. Takenaka, et al., U.S. Pat. No. 3,860,850, issued Jan. 14, 1975, teaches the use of V-shaped enhancement members located above and below three in-line beams and the use of C-shaped shields around the two outer beams. Hughes, U.S. Pat. No. 3,873,879, issued Mar. 29, 1975, teaches the use of small disc-shaped enhancement elements above and below the center beam and ring-shaped shunts around the two outer beams. Ando, et al., U.S. Pat. No. 4,142,131, issued Feb. 27, 1979, teaches magnetic pole piece plates located above and below the outer beams and between the outer beams and the center beam.
The multiplicity of different coma correcting structures, some of which may be used in combination to achieve varying amounts of distortion correction, pose manufacturing problems when electron gun mounts embodying one or more coma correcting structures are overproduced. Since the required amount of the coma correction often varies from tube-type to tube-type, it is desirable to be able to modify electron guns to achieve the desired coma correcting structure for each specific tube requirement.